This invention relates to the subject matters of Disclosure Document No. 115,952, filed Mar. 28, 1983 and Disclosure Document No. 119,694, filed Aug. 15, 1983.
1. Field of the Invention
This invention relates to a process for reducing contaminants in wastewater effluents such as those generated by the electroplating industry or by cleaning boiler tubes in the power industry. More particularly, it relates to an environmentally-compatible, economically attractive and simplified process for removing dissolved free and chelated heavy metals from aqueous streams without formation of any by-product presenting further environmental problems.
2. Description of the Prior Art
Heavy metals such as lead, chromium, copper, zinc, cadmium, and nickel are commonly found in a variety of industrial wastewater effluents. In many industries, such as electroplating, the heavy metals in the wastewater are complexed or chelated and difficult to remove by conventional processes. Conventional chelating agents include, for example, solutions of ammonium citrate, solutions of hydrazine and ethylenediaminetetraacetic acid (ammoniated EDTA), and others known to those skilled in the art.
The United States Environmental Protection Agency (EPA) has set forth certain regulations which require pretreatment of industrial wastewaters prior to discharge to a municipal wastewater treatment plant as well as requirements for industries discharging directly to public waters. The requirements are based upon recognition of the toxic effects of heavy metals on the ecosystem, particularly aquatic organisms.
As part of its regulatory procedures, the EPA undertook studies of the economic impact of these regulations and estimated the costs for various industries based on available technology. Conventional processes for removal of heavy metals involve chemical addition to precipitate the metals and unit processes to enhance flocculation and sedimentation of the precipitant. The by-products of these processes are hydrous sludges containing 95 to 97% water. They are typically classified as hazardous wastes under current EPA criteria. Thus, the cleanup of wastewaters for ecological purposes often results in the production of still another ecological problem, i.e., disposal of the sludges. Handling and disposal of such sludges present significant cost and administrative burdens to the waste generator.
The recognized need for the purification of industrial wastewater effluents is reflected in the prior art. U.S. Pat. No. 3,027,321, for example, relates to the treatment of aqueous solutions containing toxic chromate ions by passing the solutions through a ferrous metal chip bed. Carbon dioxide is dissolved into water to create an acidic condition that is highly reactive with ferrous materials. The carbonic acid-ferrous corrosion process forms a reactive ferrous bicarbonate complex which reduces the chromate ions and results in formation of inert hydroxides of iron and chromium as precipitates which are drained off. These precipitates must then be disposed of in an environmentally acceptable manner.
U.S. Pat. No. 3,697,567 discloses a specific process for removing dissolved organic lead from an aqueous effluent produced in the manufacture of alkyl lead compounds by contacting the effluent with a metal more electropositive than lead but essentially non-reactive with water, e.g., iron, zinc, aluminum and magnesium. At least a portion of the dissolved organic lead is converted to an insoluble lead-containing product. However, this process does not address the problem of removing heavy metals that are more electropositive than the metal with which the waste is contacted.
U.S. Pat. No. 3,922,224 teaches a wastewater treating process based on electrodeposition of the toxic metal on the iron and release of iron ions which are subsequently precipitated. The effluent containing toxic heavy metals which are to be removed is passed through a bed of iron turnings, mixed with sand, the bed being agitated mechanically by a current of air bubbles. The iron is dissolved in the aqueous environment while the toxic metals are deposited on the surface of the iron and then detached from the iron by mechanical abrasion of the sand. The toxic metals are separated and recovered in the form of a metallic powder.
U.S. Pat. No. 3,960,723 relates to a process for treating aqueous waste solutions containing complexing or oxidizing agents and dissolved metallic compounds of copper, chromium, nickel, lead or tin. The waste solution is treated with finely-divided magnetized ferrous particles which are uniformly stirred, followed by the addition of material to increase the pH. The resulting metal precipitates are settled out as a sludge and separated from the waste stream for disposal. Having solved one environmental problem, disposal of the sludge creates another.
U.S. Pat. No. 4,343,706 teaches removal of various heavy metals from industrial waste streams by flocculation with ferric ions and an alkaline material at a basic pH. The flocculated heavy metals and coagulant chemicals are recovered under acidic conditions. The recovered heavy metals by this process are in a concentrated aqueous waste stream which requires further processing, still another environmental problem.
As above indicated, the primary mechanisms for heavy metal removal in these prior art processes are precipitation as an inert hydroxide and electrodeposition. The continued concern about the fate of toxic heavy metals in the environment has also led to study of the adsorptive capacity of what is believed to be ferric oxyhydroxide as a medium for removing complexed heavy metals from aqueous streams. The adsorptive capability of ferric oxyhydroxide is well documented by these studies. Favorable results have been reported, including the experimental removal of cadmium, nickel, lead, zinc and titanium. These studies are based, for example, on preparation of an hydrous metal oxide by adding a metal (iron or manganese) nitrate solution to an alkaline solution or by using commercially available goethite.
While the adsorption approach has proved encouraging, the ferric oxyhydroxides formed in these laboratory studies are in the form of dilute suspensions which, after adsorption of the heavy metals thereon, must be settled out or otherwise isolated for recovery. Moreover, a low-cost, practical embodiment thereof which uses readily available treating materials without producing a new environmental by-product problem has proved elusive.
Furthermore, any approach must lend itself to compactness, simplicity and retrofitting to small industrial units, such as small electroplating plants, which may lack adequate treating facilities and staff, usually have little sophistication in such matters and have limited financial resources to devote to the environmental consequences of their operations. Still further, because toxic streams are produced either intermittently or continuously, any treating process must also lend itself to either batch or continuous treatment. To date, these needs have not been satisfactorily fulfilled.